Hi Geek99, thanks for replying.
The stack knob jazz circuit (below) is actually quite a bit different to the Hofner. Only two lugs of each tone pot are connected - the signal goes to the middle wiper lug, then through the cap to ground via the variable resistor. The third lug (the other end of the variable resistor) is left floating. With this configuration, even with the tone fully rolled off, the pot resistance doesn't impede the main signal path.
With the Hofner circuit, when the tone is fully rolled off the full pot resistance is added to the signal path. The Hofner circuit also has 2 additional caps compared to the stack knob circuit and the main tone caps (in this case 100n and 47n) aren't where you'd expect them to be.
I've checked the wiring of the bass itself (which is original) against the Hofner circuit diagram and it matches, so that's how they intended it to be. It just doesn't function as you'd expect a regular tone circuit to function. It's all a bit odd!